Filming methods utilizing solvent-based filming lakes are known, which, when deposited on a humid phosphor layer, produce a thin and continuous film on which a metal layer condenses by sputtering.
Such processes produce handmade articles having good characteristics, however they have the drawback of requiring the employment of solvent which must be subsequently evaporated and recovered to avoid environmental pollutions. The production plants need therefore units for the solvent recovery.
For this reason films deriving from drying of aqueous emulsions have been studied since long.
On the panel carrying the screen, the aqueous emulsion deposits, which is dried by heating.
The purpose of this polymeric film is to remove the irregular surface of the phosphor particles forming the screen, and to provide a very uniform sublayer on which the sputtered metal then deposits in order to form a surface like a mirror to reflect the screen light.
Thanks to this polymeric film the metal does not penetrate into the screen below even though the phosphor particles can jut outside the film.
The screen then undergoes a treatment at high temperature from 400-450.degree. C. (the so called baked-out) to remove the polymeric film. The gaseous products of the polymeric film decomposition come out of the holes of the metal layer.
A drawback of this process consists in that the carbon residues deriving from the film decomposition lead to an insufficient gloss of the screen.
These processes require therefore an additional process of slow prebaking to completely remove all the organic materials.
Processes wherein sufficient amounts of air are used during baking to volatilize the acrylic polymer, are also known. See or instance U.S. Pat. Nos. 4,217,015 and 4,350,514.
This needs process changes for introducing air, leading to more expensive and complex processes.
Aqueous emulsions based on acrylic polymers capable of forming films for preparing cathode tubes are described in U.S. Pat. No. 5,145,511. It is a process for manufacturing cathode tubes with only one step of film removal deriving from the emulsion simultaneously with the sealing of the cathode tube panel.
To this purpose it must be noticed that any filming acrylic emulsion cannot be employed in the process of the patent indicated from an industrial point of view. In fact tests carried out by the Applicant (see the examples hereafter) have shown that the emulsions based on acrylic esters utilized in said patent do not succeed in meeting industrial requirements, i.e., in useful times, compared with conventional processes. The necessary times for having the film decomposition are high. As a matter of fact said tests have shown that the decomposition of said acrylic polymers leaves also considerable organic residues, from about 0.5-2% by weight which are detrimental for the screens gloss.